Star trackers are typically used on-board spacecraft as a means for providing data for adjustment of the attitude of the spacecraft in space. The star trackers provide positional reference data to the attitude sensing system of the spacecraft so that the attitude of the spacecraft may be adjusted and controlled. Star trackers utilizing charge-coupled-device (CCD) arrays for obtaining star image sense position with respect to two axes which are orthogonal to the line-of-sight vector of the star tracker (line-of-sight vector is normal to the CCD array).
Heretofore, the line-of-sight orientation of star trackers installed on-board spacecraft are typically aligned with and use the spacecraft reference axes, such as the roll, pitch, or yaw axes, as the reference axes for the tracker. Because the CCD arrays can only acquire data with respect to two dimensions, the spacecraft reference axis along the CCD array line-of-sight vector, which is normal to the array, has degraded accuracy. For example, star trackers utilizing the pitch and yaw reference axes of the spacecraft as reference axes for the tracker have degraded accuracy of roll knowledge about the roll axis of the spacecraft. This degraded accuracy about the roll axis is due to the fact that the tracker's line-of-sight vector and the roll axis are identical and normal to the CCD array, and thus the tracker has poor sensitivity along the roll axis. To compensate for this insensitivity, a second star tracker is necessary to provide data to the attitude control system about the roll axis (the third reference axis) so as to give the spacecraft attitude system accurate position information with respect to all three spacecraft reference axes, and, thus, accurate attitude data.
The system and method of the present invention improves the sensitivity of the star tracker with respect to the third spacecraft axis and, thus, improves the accuracy of the spacecraft attitude control system by providing an on-board star tracker having equal pointing error about all three axes of the spacecraft. The present invention also provides the advantage of less weight on-board the spacecraft because, in some instances, attitude control may be accomplished with a single tracker rather than two.